The increasing demand to improve vehicular fuel economy and reduce vehicular emissions has led to the development of both hybrid vehicles and pure electric vehicles. Pure electric vehicles may be powered by a battery pack (which is made up of numerous smaller modules or cells), while hybrid vehicles include two or more energy sources, such as a gasoline (also referred to as an internal combustion) engine used as either a backup to or in cooperation with a battery pack. There are two broad versions of hybrid vehicles currently in use. In a first version (known as a charge-depleting hybrid architecture), the battery can be charged off a conventional electrical grid such as a 120 VAC or 240 VAC power line. In a second version (known as a charge-sustaining hybrid architecture), the battery receives all of its electrical charging from one or both of the internal combustion engine and regenerative braking. In either configuration, various parameters associated with the battery pack can be monitored to ensure proper operation.
The temperature of a cell in a battery (or battery pack) is a critical factor related to the life and performance of the battery. Thus, the accurate determination of the temperature of a cell is essential to the life and performance of a battery. While one method of determining the temperature of a cell in a battery is to put a sensor in contact with the core of the cell in a battery, this method is generally available only in a laboratory setting, as it is placed in a sealed area of the battery that would be inaccessible in a production environment. As a result, the temperature of a cell in a battery is currently determined by directly measuring the temperature of the surface of the cell with a sensor. However, the temperature of the surface of a cell in a battery is often different from the temperature of the core of a cell in a battery. For example, the difference in temperature between the surface temperature of a cell and the core temperature of a cell in a battery during an operative period may be as large as 10° C. A difference in temperature of 10° C. can significantly alter the life and performance of a battery. For instance, a battery can be operated for greater than twelve years if the battery functions at 25° C.; however, a battery may be operated for only seven years if the battery functions at 35° C. Additionally, a battery can provide about 14 kW of power at 0° C.; however, a battery can provide only about 8 kW of power at −10° C. As a result, the use of the surface temperature as a representation of the core temperature may introduce error into calculations involving the core temperature of a cell in a battery and may also have a significant impact on the life and performance of the battery.